Anti-skid control apparatus for a vehicle braking system

ABSTRACT

The present invention related to an improved antiskid control apparatus. When the front wheel on either the right and left sides becomes subject to antiskid control, this apparatus detects whether or not the road surface has a split μ condition in which the road surface friction coefficient at the right and left wheels differs by more than a prescribed value. When the road surface is determined to be in a split μ condition, the braking power for the rear wheels is lowered stepwise during antiskid control to suppress spinning and secure a large side force. This side force cancels the yaw moment which is generated by the difference of friction coefficients in the running direction on the right and left wheels.

BACKGROUND OF THE INVENTION

The present invention relates to an antiskid control apparatus and morespecifically an antiskid control apparatus that improves stability undersplit μ road surface conditions where the coefficients of friction ofthe road surfaces, upon which the right and left wheels make contact,differ greatly from one another.

PRIOR ART

Antiskid control apparatuses that prevent wheel lock during vehiclebraking are known in the art and have been previously disclosed, forexample, in Japanese Patent Application, 2nd publication, No. 56-28738and No. 59-19863.

In the antiskid control apparatuses disclosed therein, the presence orabsence of the tendency for each wheel to lock is based upon themeasured value of the wheel speed, and the locking of any wheel isprevented by controlling the rise in the fluid pressure of the hydraulicpiping system of the wheel.

As a result of ice on one portion of a road the friction coefficient ofthe road surface with which the right wheel makes contact differsgreatly from that with which the left wheel makes contact. In such acase, countermeasures are taken against vehicular spin caused by anunequal braking force between these two wheels. That is, the rise in thebrake fluid pressure of the wheel opposing the paired wheel which isantiskid-controlled (for example, the right front wheel opposing theleft front wheel, or the right rear wheel opposing the left rear wheel)is controlled and a sudden occurrence of the yaw moment, which causesthe spin, is controlled. In this manner, the driver is allowed asteering margin ensuring driving stability.

The above-mentioned spin countermeasures have been developed primarilyfor full-sized vehicles (in general, long wheelbases are best-suited tononswerving forward motion during braking). Therefore, a problem thatremained was that antiskid apparatuses could not be directly applied tocompact vehicles though the tendency is to equip these vehicles withantiskid control apparatuses in recent years.

In general, compact vehicles which have short wheelbases are inferior innonswerving forward motion during braking when compared with full-sizedvehicles that have long wheelbases. In addition, the load on the rearwheels of compact vehicles, that have an important role in maintainingdirect nonswerving forward motion, is low. As a result, in theabove-mentioned conventional antiskid control, there are situationswhere spin countermeasures achieved by just lowering the speed at whichthe yawing moment occurs cannot ensure sufficient driving stability.

SUMMARY OF THE INVENTION

The object of the present invention is to provide reliable antiskidcontrol which ensures driving stability under split μ road surfaceconditions regardless of the type or size of vehicle to which it isapplied.

In order to accomplish the above-mentioned object, in an antiskidcontrol apparatus which is provided with wheel-speed sensors whichdetect the speed of each wheel, a modulator which selects and sets oneof three modes--an increase pressure mode which raises the brake fluidpressure of each wheel, a decrease pressure mode which controls theincrease in brake fluid pressure, and a hold mode--and a controllerwhich, by determining whether or not the wheel speed data detected byone of the wheel speed sensors exceeds a prescribed threshold value,determines whether or not the corresponding wheel is going to lock and,if so, controls the mode of the modulator in such a manner that theantiskid control is initiated by setting either the decrease pressuremode or the hold mode to control the increase in brake fluid pressure ofthe wheel that is going to lock; the controller is characterized inbeing equipped with a split μ detector which, when the front wheel oneither the right or left sides is antiskid controlled, detects whetheror not there exists a road surface condition of split μ in which thefriction coefficients of the road surface in contact with the right andleft wheels differ from each other by a prescribed value or more, and abraking power controller which, upon detecting the condition of split μby means of the split μ detector, decreases the braking power of therear wheels continuously during antiskid control.

In a split μ condition where the wheel on either one of the right andleft sides makes contact with a low μ road surface and the wheel on theother side makes contact with a high μ road surface, when the frontwheel on the former side has a tendency to lock, and the antiskidcontrol is initiated and, at the same time, the split μ road conditionis detected, decreasing the braking power of the rear wheel, to suppressspin generation, and decreasing the braking force of the rear wheelcontinuously during antiskid control to ensure a large side force. Thisside force can cancel the yaw moment generated by the difference infriction coefficients in the forward direction between the right andleft wheels and thereby prevent the vehicle from spinning. This spinprevention effect is especially large when there is a large side forceon the rear wheel during a braking operation in which the center ofgravity is shifted forward.

A BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a control flowchart on spin prevention.

FIGS. 2(a), 2(b), 2(c), 2(d), and 2(e) shows changes in wheel speed,front wheel braking pressure, rear wheel braking power, split μ counter,and the threshold pressure when depressurizing the rear wheel during thecontrol.

FIGS. 3(a), 3(b), 3(c) and 3(d) shows changes in wheel speed, frontwheel braking pressure, split μ counter, and the threshold pressure whendepressurizing the rear wheel in cases where the braking pressure's rateof increase is different between the right and left wheels.

FIG. 4 is a piping diagram showing the brake fluid pressure system.

FIG. 5 is a piping diagram showing the constitution of the modulator.

FIGS. 6(a), 6(b), and 6(c) shows the changes in wheel speed, front wheelbraking pressure, and rear wheel braking pressure each during antiskidcontrol by means of conventional apparatus.

FIG. 7 shows the relationship of coefficients of road surface frictionand side force to slip ratio.

FIG. 8 is a diagram showing the relationship between the yaw momentacting on a vehicle during braking and the side force working againstit.

FIG. 9 is a flowchart showing control in the second embodiment accordingto the present invention.

FIG. 10 is a control flowchart of the third embodiment of the presentinvention.

FIGS. 11(a), 11(b), 11(c), 11(d), 11(e), and 11(f) changes in wheelspeed, front wheel braking power, rear wheel braking power, split μcounter, threshold pressure when depressurizing the left front wheel,and threshold pressure when depressurizing the rear wheel.

FIG. 12 is a plan view similar to FIG. 8 in the third embodiment.

FIG. 13 is a flowchart of the fourth embodiment of the presentinvention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments in accordance with the present invention willbe explained with reference to the figures.

The entire system of the antiskid control apparatus will be explainedwith reference to FIGS. 4 and 5.

Numeral 1 is a master cylinder, which generates brake fluid pressure bymeans of the driver's stepping force on the brake pedal 2. This fluidpressure produced by this master cylinder 1 is supplied to the wheelcylinders of the right and left front wheel brakes 4 and 5 throughmodulators 3 and fluid pressure is also supplied to the wheel cylindersof the right and left rear wheel brakes 7 and 8 through fluid pressurecontrol valves 6. The modulators 3 are provided in the hydraulic systemleading to the respective wheel cylinders 4, 5, 7 and 8 perform thefunctions of controlling the rise in brake fluid pressure or recoveringthe brake fluid pressure by means of a control signal supplied from thecontroller 9. The details of the modulators 3 will be described later.The above-mentioned fluid pressure control valves 6 are provided in thehydraulic system leading from the master cylinder 1 to the rear wheelbrakes 7 and 8 and limits the fluid pressure in each hydraulic system toa level below that of the fluid pressure in the hydraulic systems forthe front wheel brakes 4 and 5. In this way, the front wheels tend tolock taking precedence over the rear wheels during braking. Wheel speedsensors S, that determine the speed of each above-mentioned wheel, areprovided in each wheel. A control signal is supplied to modulator 3 fromthe above-mentioned controller 9 based on the wheel speed data obtainedby these wheel speed sensors S Prescribed antiskid control is performedby means of this control signal.

Next, the composition of the embodiment of each of the above-mentionedmodulators 3 will be explained according to FIG. 5.

Numeral 10 is a switch valve that can be set to open and closedpositions. The hydraulic system leading to each wheel cylinder 4, 5, 7and 8 from the master cylinder 1 can be opened and closed by means ofthis switch valve 10 A check valve 11 is attached to the above-mentionedswitch valve 10 and allows fluid to flow in the direction toward themaster cylinder 1 when the switch valve 10 is in the "closed" position.A switch valve 12 is provided at a position parallel to that of theabove-mentioned switch valve 10. This switch valve 12 opens and closesby means of a control signal supplied from the above-mentionedcontroller 9 to discharge fluid pressure inside wheel cylinders 4, 5, 7and 8 into reservoir 13.

Numeral 14 indicates a pump that is driven by motor 15. This pump 14 isdriven, based on a control signal supplied from the above-mentionedcontroller 9, to recover the fluid pressure that was reduced in thehydraulic system at the time of antiskid control. By using theabove-mentioned configuration, modulator 3 is selectively set at any ofthe following configuration, a, b, and c.

a. When switch valve 10 is set to "open" and switch valve 12 is set to"closed", modulator 3 is in the increase pressure mode.

b. When switch valve 10 is set to "closed" and switch valve 12 is set to"open", modulator 3 is in the decrease pressure mode.

c. When both switch valve 10 and switch valve 12 are set to "closed",modulator 3 is in the hold mode.

The flow of the fluid in the piping between each wheel cylinder 4, 5, 7and 8 and master cylinder 1 is controlled in the above-mentioned way.

The antiskid control that performs by means of the above-mentionedcontroller 9 computes, upon receiving a signal supplied from each wheelspeed sensor 8, the wheel speed Rω or the acceleration Rω/dt has anegative value during deceleration which is the differential value ofthe wheel speed. Furthermore, it computes the slip ratio 1 from theabove-mentioned wheel speed Rω and the simulated vehicle speed V, whichis obtained from the change curve of wheel speed Rω during deceleration.For example, based on either acceleration or deceleration Rdω/dt and theslip ratio 1, breaking condition as indicated in Table 1 and statedhereafter is applicable to the present braking conditions, modulator 3is set to the increase pressure mode, decrease pressure mode or holdmode.

In other words, when the slip ratio 1 increases past a prescribedthreshold value, or the wheel speed decreases past a prescribedthreshold value, the brake fluid pressure decreases, and further, whenthe slip ratio 1 is below a prescribed threshold value and the wheelspeed has a tendency to accelerate, the brake fluid pressure increases.

                  TABLE 1                                                         ______________________________________                                                             increase                                                                             increase                                                                             increase                                                                             increase                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          -A                                                                                   increase                                                                             increase                                                                             hold   increase                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            wheel   O                                                                     de-                  increase                                                                             hold   decrease                                                                             decrease                            celeration           pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          +B                                                                                   hold   decrease                                                                             decrease                                                                             decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            ______________________________________                                         → slip ratio                                                      

As both A and B represent positive values, in the case wheredeceleration is represented by -A, the wheel speed increases at anacceleration of +A.

When the wheel speed is increasing at an acceleration exceeding aprescribed value A, according to the conditions shown in theabove-mentioned Table 1, the brake fluid pressure is raised irrespectiveof the slip ratio based on the determination that the wheel speed Rω hasa tendency to recover. When the wheel speed falls at a decelerationexceeding a prescribed value B, the decrease pressure mode is selectedbased on the determination that the wheel speed is approaching a lockcondition.

In other words, the suitable mode of operation is selected according tothe determination that (See in Table 1) a dangerous condition in whichthe tendency to lock is developing as designated in the right lower partof the table or a safety condition which departs from the tendency tolock is developing as designated in the upper left part of the table.

In addition, the fluid pressure of the rear-wheel wheel cylinders 7 and8 can be controlled separately by their respective modulators. However,in this case, the fluid pressure is controlled at an equal level by the"select-low control" that gives fluid pressure priority to the rearwheel which has a tendency to lock.

Next how the above-mentioned controller components perform spincountermeasures during braking will be explained with reference to theflowchart shown in FIG. 1.

Step 11

A determination is made as to whether or not one of the front wheels (inthis embodiment, the right front wheel) is being antiskid-controlled. Incase of "YES" the controller proceeds to step 12. The controller willproceed to step 21 in the case of "NO". The determination as to whethera wheel is being antiskid-controlled or not is carried out for eachwheel. When the modulator of one wheel enters into the decrease pressuremode, the flag which indicates that this wheel is beingantiskid-controlled is set. The determination is made by checking thisflag.

This flag is reset to the initial state by stopping the switch signal bystepping on the brake pedal or when the speed signal stops from thewheel speed sensor S in response to stopping the vehicle. It ispermissible to set the under-antiskid-control indicator flag when thecontroller enters the hold mode as well as the decrease pressure mode.

Step 12

A determination is made whether or not the other front (in thisembodiment, the left front wheel) is being antiskid-controlled. In thecase of "NO", the controller proceeds to the following step 13; in thecase of "YES", the controller proceeds to step 19 described hereinafter.

Step 13

A determination is made as to whether or not the depressurizing periodfor the right front wheel has exceeded a prescribed value. In the caseof "NO", the controller proceeds to step 14; in the case of "YES", itproceeds to step 15. Here, a long depressurizing interval describes ahigh degree of split μ. The purpose of step 13 is to detect apreliminary value for split μ and, based on the results, to define thecontrol components for the left front wheel according to the conditionsof the right front wheel. Therefore step 13 is called a preliminarydetermination step based on the consideration of its function.

Step 14

When "NO" is determined in step 13, the switch valve 10 isintermittently opened and closed to pulsively raise the fluid pressureof the left front wheel and thereby lower the increasing rate of brakingforce of the above-mentioned left front wheel. In addition, when theincreasing rate has been lowered, this condition is maintained as it is.

Step 15

When "YES" is determined in step 13, a determination is made as towhether or not the right front wheel is being depressurized. In the caseof "NO", the controller proceeds to step 14 and in the case of "YES", itproceeds to the next step 16.

Step 16

The controller proceeds to step 17 after reducing the brake fluidpressure of the left front wheel. This step takes the following matterinto consideration. When depressurization continues past a prescribedperiod of time the controller lowers the difference between the brakingforces of two wheels by forcibly depressurizing the left front wheelwhich is not antiskid-controlled. The function included in both thisstep 16 and the above-mentioned step 14 is called brake suppressor.

Step 17

A determination is made as to whether or not the split μ counter of theleft front wheel has exceeded a prescribed value. In the case of "YES",the controller proceeds to the next step 18 and in the case of "NO",control stops.

Values in the split μ counter can be obtained counting the number ofpulses while the fluid pressure is increasing as in step 14 or bytotaling the number and the depressurizing time in step 16 in terms ofthe count value which is obtained by using a prescribed conversionfactor, such as a certain span of time, for example, 5 ms, per onecount. Then a determination is made as to whether or not the count hasexceeded a prescribed value. The split μ, which means that thecoefficient for the friction of the road surface is different betweenthe right and left wheels, is detected by when the count exceeds aprescribed value. This function is called split μ detector.

Step 18

The threshold value (of the slip ratio and/or wheel deceleration) whendepressurizing the real wheel for antiskid control. When the thresholdvalue has been previously lowered, the value is maintained as in theprevious step. The function of this step 18 is called a threshold valuechanging step.

Step 19

While the above-mentioned steps 13-17 are constituted such that adetermination can be made as to whether the threshold value whendepressurizing the rear wheel in the first cycle of antiskid controlmust be lowered is made in step 17. If the value is lowered in step 18,this step 19 and the succeeding step 20 are necessary to determinewhether or not to lower the threshold value in the cycle for the leftfront wheel during antiskid control.

In step 19, a determination is made as to whether the split μ counter ofthe left front wheel has exceeded a prescribed value and in case of"YES", the controller proceeds to step 18 while, in case of "NO", thecontroller proceeds to the succeeding step 20. When step 19 provides thesame determination that the split μ counter has exceeded a prescribedvalue made in step 17, the controller proceeds to step 18. When adetermination that the split μ counter has not exceeded the prescribedvalue is made in step 17, the controller proceeds to step 20 for thesame reason as in step 19.

Step 20

Contrary to step 19, a determination is made as to whether the split μcounter of the right front wheel has exceeded a prescribed value. In thecase of "YES", the controller proceeds to step 27 as will be explainedlater. In the case of "NO", one control cycle is finished.

Step 21

When the right front wheel is not undergoing antiskid control, adetermination is made as to whether or not the left front wheel isundergoing antiskid control. In the case of "YES", the controllerproceeds to step 22 to provide the right front wheel with the samecontrol as described in the above-mentioned steps 13-18. In the case of"NO", the controller proceeds to step 28.

Step 22

A determination is made as to whether the depressurizing time of theleft front wheel has exceeded a prescribed value. In the case of "NO",the controller proceeds to step 23 and, in the case of "YES", to step24.

Step 23

By pulsively raising the brake fluid pressure of the left front wheel,the increasing speed of the braking force for the right front wheel islowered. In addition, when the increasing speed of the braking force hasbeen previously lowered, this setting is maintained as it is.

Step 24

A determination is made as to whether the brake is being depressurized.In the case of "NO", the controller proceeds to step 23 and in the caseof "YES", to the next step 26.

Step 25

The brake fluid pressure of the right front wheel is reduced.

Step 26

A determination is made as to whether or not the split μ counter of theright front wheel has exceeded a prescribed value. In the case of "YES",the controller proceeds to the next step 27 and in the case of "NO" thecontrol cycle is finished.

Step 27

The split μ counters of the right and left wheels are cleared.

Step 28

The split μ counters of both the right and left wheels are cleared.

Next, the behavior of an antiskid-controlled vehicle under theconditions of split μ where the coefficients of friction between theright and left wheels differ from each other is explained with referenceto FIG. 6. The road surface conditions shown in FIG. 6 are such that theright side of the road surface is low μ (for example, an icy road) andthe left side is high μ (for example, a dry asphalt road).

In a conventional antiskid control that controls the rate of theincrease in the braking power of the front wheel on the opposite side ofone of the front wheels which is undergoing antiskid control, and whenthe antiskid control is started by detecting an abrupt reduction of thewheel speed of the right front wheel as shown FIG. 6(b), the brake fluidpressure of the left front wheel is gradually increased in a stepwiseform, as shown by numeral 31, to lower the rate of the increase in thebraking force. However, in the right front wheel and right rear wheel,the brake pressure rises at a high speed in accordance with the pressureon the brake pedal 2 as shown in FIGS. 6(b) and (c), resulting in astrong slipping tendency as shown in FIG. 6(a).

In general, a relationship exists between the tires and the road surfaceas shown in FIG. 7. The coefficient of friction (road surface μ) in thedirection of the motion of a tire reaches a peak value in the rangewhere the slip ratio is 0.05 to 0.3, but the friction coefficient(hereinafter called sideforce coefficient) in the directionperpendicular to the rolling direction of the tire tends to graduallydecrease as the slip ratio 1 increases. Accordingly, at the time pointof X in FIG. 6, the braking force as indicated by the arrows 40a, 40b,40c and 40d (appended letters of a to d indicate the left front-, rightfront- left rear-, and right rear wheels respectively) in FIG. 8 aregenerated in each wheel. These forces 40a to 40d working generate a yawmoment shown by arrow 41 near the center of gravity 42 of a vehicle.When the moment formed by the side forces (shown by arrows 43a to 43d inFIG. 8) is not sufficient to oppose this yaw moment, the vehicle willspin. However, the conventional control system shown in FIG. 6 whichonly delays the rise in brake fluid pressure of the wheel on the sidenot being antiskid controlled only delays the generation of the momentas shown by arrow 41. Hence, if the proper operation is not executed bythe time this moment is generated, spin may occur. The possibility ofspin is particularly high in compact vehicles because the wheelbase ofthese vehicles is short and the absolute value of friction of the wheelsagainst the road surface is small due to a small load on the rearwheels, and accordingly, the side forces shown by arrows 43c and 43d aretoo weak to cope with the yaw moment 41.

On the contrary, in the control system of the present embodiment, whenthe right or left front wheel starts to be antiskid-controlled and itsdepressurizing time exceeds a prescribed value, the other front wheel onthe side opposite the wheel under antiskid control is pulsivelypressurized or depressurized to decrease the difference in the brakingforces of the two wheels in response to the control conditions of thewheel under antiskid control.

If the forced control of the other front wheel continues for aprescribed period of time, that is, when the split μ counter hasexceeded a prescribed count value, it is determined whether the split μexists or the wheel has a high μ, and then the threshold value whendepressurizing the rear wheel in the antiskid control is lowered topromote the start of antiskid control. Consequently antiskid control canbe easily applied to the right rear wheel. Furthermore, even afterantiskid control of the other front wheel is initiated, when the split μcounter of each front wheel exceeds a prescribed value, the thresholdvalue when depressurizing the rear wheel is lowered or left as is (whenthe threshold has been previously been lowered) to lower the brakingpower of the rear wheel continuously during antiskid control. In otherwords, although conventional control increases the brake pressure in theright rear wheel (as shown by a broken line in FIG. 2(c)), controlaccording to this application suppresses this increase (as shown by asolid line), thus facilitating antiskid control and decreasing brakingpower.

This process may be explained in terms of the threshold value of theslip ratio. When the threshold value shown by A' in FIG. 7 is used inthe control, the control region of the vehicle is (3) as a result ofovershooting due to the inertia of the vehicle. However, when thethreshold value is changed to B', the control region is changed toregion (4). Thus, after the time indicated by mark Y in FIG. 2, thebraking force of the rear wheel is lowered as shown by mark 44c and 44din FIG. 8 (a dotted and dashed line), yielding a yaw moment (45) smallerthan the previous yaw moment (41) and making it difficult for a vehicleto spin. Meanwhile, since the side force coefficient is increased by thechange of threshold from A' to B', the side force of each rear wheel isstrengthened as shown by marks 46c and 44d, and furthermore it cancancel the yaw moment (45).

As described above, the threshold value can be changed in order toreduce the braking force of the rear wheel and increase the side forcein consideration of the inertia of the vehicular system, and the sideforce and braking force can be set so as to equally or a little moreincrease.

In consideration of inertia of the vehicular system, the threshold valuecan be changed as described above so as to reduce the braking force ofthe left front wheel and increase the side force. The braking force canbe made equal to or greater than the side force. The split μ counter maybe applied to two kinds of measurements. One counts the number of pulsesthat increment the brake fluid pressure as indicated by (2), and theother counts the sum of the above-mentioned increments and thedepressurizing time in terms of the increments indicated by (2). Whereasthe split μ counter cannot detect split μ in some cases where thecounter or does not exceed the prescribed value as shown in FIG. 3(c).

Second Embodiment

The flow chart of the second embodiment of this invention is shown inFIG. 9. Only differences from FIG. 1 will be explained.

Step 51

The split μ counter of the right front wheel is cleared.

Step 52

The threshold value for depressurizing the rear wheel in the fist cycleis decreased for antiskid control of the left front wheel.

Step 53

The threshold for depressurizing the rear wheel in the usual cycle isdecreased for antiskid control of the left front wheel.

Step 54

The split μ counter of the left front wheel is cleared. Thus, whereasthe amount that depressurization will be decreased in the antiskidcontrol of the rear wheel must be identical when the lowering isdetermined in the first cycle and the usual cycles in the firstembodiment, the amount to be lowered can be separately set for the firstcycle and usual cycle. Consequently, by further decreasing the thresholdvalue in depressurization for the first cycle, earlier antiskid controlcan be initiated.

Step 55

The split μ counter of the left front wheel is cleared.

Step 56

The threshold value in the depressurization of the first cycle islowered in the right front wheel.

Thus, whereas the amount to be lowered in depressurization in antiskidcontrol is set identical in both cases where the lowering is determinedin the first cycles and the usual cycles in the first embodiment, theamount to be lowered can be set separately in the cases of the firstcycle and the usual cycle. Consequently, by giving a larger setting tothe amount to be lowered in the threshold value in depressurization forthe first cycle, antiskid control can be initiated earlier.

The Third Embodiment

The third embodiment of the present invention is shown in FIGS. 10-12,and only its differences from the first embodiment are explained withnew marks.

The flowchart in FIG. 10 includes the following steps.

Step 61

The threshold value for depressurizing the left front and rear wheels islowered for antiskid control. If previously lowered, the threshold doesnot change.

Step 62

The threshold value for depressurizing the right front and rear wheelsis lowered.

In other words, in this embodiment, when either the right or left frontwheel becomes subject to antiskid control, the threshold value will belowered for the other front wheel as well as for the rear wheels. Thiscan be explained by applying the slip threshold in depressurization asfollows. When the threshold value is changed from A' to B' in FIG. 7,the braking power is diminished for the left front wheel as well as forthe rear wheels as shown by 63a in FIG. 12. This changes the yaw moment(41) to a diminished yaw moment (64) and reduces the possibility of thevehicle spinning. Moreover, the side forces, increased by the change ofthreshold from A' to B' as shown by FIG. 12 65a to 65c, can cancel theyaw moment.

The Fourth Embodiment

FIG. 13 shows the fourth embodiment of the present invention. Thisembodiment sets the amount of decrease in the threshold in the firstcycle of antiskid control differently from the amount of decrease in theusual cycle (as in the second embodiment) compared with the thirdembodiment shown in FIG. 10. Earlier antiskid control can be started byfurther decreasing the threshold for depressurizing in the first cycle.

Only those components that differ from the flowchart of FIG. 10 areexplained hereunder with new marks.

Step 71

The split μ counter of the right front wheel is cleared.

Step 72

In the first cycle of antiskid control of the left front wheel, thethreshold for depressurizing the left front and rear wheels is lowered.

Step 73

In the usual cycle of antiskid control of the left front wheel, thethreshold for depressurizing the left front and rear wheels is lowered.

Step 74

The threshold for depressurizing the left front and rear wheels islowered in the usual cycle of antiskid control.

Step 75

The split μ counter of the left front wheel is cleared.

Step 76

In the first cycle of antiskid control of the right front wheel, thethreshold for depressurizing the right wheel and rear wheels is lowered.

Modified Embodiment

(1) It may be possible to control the rear wheels in a similar way tothe front wheel on the low μ side so as to reduce the braking power andside forces on the rear wheel sides.

(2) In the above-mentioned embodiments, an explanation is given of thecase where switching is made between "increase pressure", "hold" and"decrease pressure" in accordance with the condition in Table 1.Needless to say, this invention can be used for switching conditionsother than those mentioned above. As for the threshold value, thepurpose is ensure the prescribed side force can be accomplished in bothcases where either the threshold value of wheel deceleration or slipratio (or its absolute amount) shown in the Table 1 are lowered. In thiscase, for example, the threshold values giving the criteria of each modeselection in Table 1 are converted to those shown in Tables 2-4 below.

                  TABLE 2                                                         ______________________________________                                                             increase                                                                             increase                                                                             increase                                                                             increase                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          -A                                                                                   increase                                                                             increase                                                                             hold   decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            wheel   O                                                                     de-                  increase                                                                             hold   decrease                                                                             decrease                            celeration           pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          +B                                                                                   hold   decrease                                                                             decrease                                                                             decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            ______________________________________                                         → slip ratio                                                      

                  TABLE 3                                                         ______________________________________                                                             increase                                                                             increase                                                                             increase                                                                             increase                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          -A                                                                                   increase                                                                             increase                                                                             hold   decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            wheel   O                                                                     de-                  increase                                                                             hold   decrease                                                                             decrease                            celeration           pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          +B                                                                                   hold   decrease                                                                             decrease                                                                             decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            ______________________________________                                         → slip ratio                                                      

                  TABLE 4                                                         ______________________________________                                                             increase                                                                             increase                                                                             increase                                                                             increase                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          -A                                                                                   increase                                                                             increase                                                                             hold   decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            wheel   O                                                                     de-                  increase                                                                             hold   decrease                                                                             decrease                            celeration           pressure                                                                             pressure                                                                             pressure                                                                             pressure                                          +B                                                                                   hold   decrease                                                                             decrease                                                                             decrease                                                 pressure                                                                             pressure                                                                             pressure                                                                             pressure                            ______________________________________                                         → slip ratio                                                      

In Table 2, the threshold values for entry to the decrease pressure modeare set low, and entry itself is thereby facilitated for both wheeldeceleration and slip ratio. Also in Tables 3 and 4, the same principlesapply regarding slip ratio and wheel deceleration, respectively.

Since, in Table 2, the threshold value conditioned on entry into thedecrease mode is set low, the condition on entry to the decrease mode isavailable regarding both wheel deceleration and slip ratio.

Since in Table 3 the threshold value for slip ratio is set low, theconditions on entry to the decrease pressure mode is available for thisslip ratio.

Since also in Table 4 he threshold value for the wheel deceleration isset low, the conditions on entry to wheel deceleration is mitigatedregarding this wheel deceleration.

(3) While, in the above-mentioned embodiment, the threshold value indepressurization is set low in steps 18, 27, etc. in order to lower thebraking force, this objective can be realized by adopting some means ofdecreasing the braking power in the rear wheel, by, for example, slightsynchronized depressurization for the rear wheel with depressurizationof the front wheel that is subject to antiskid control, forceddepressurization of the rear wheel whenever a prescribed period of timehas passed, depressurization of the rear wheel so that the computedvalue of wheel deceleration does not exceed a prescribed value, delayingthe end of depressurization of the rear wheel, etc.

(4) In the above-mentioned embodiment, the threshold in depressurizationis set low in steps 18 and 27 so as to lower the difference betweenbraking forces. This may be achieved by lowering the braking power onthe high μ side using some method of braking power control such asslight depressurization for the wheel on the high μ side synchronizedwith the depressurization of the wheel on the low μ side under antiskidcontrol; forced depressurization of the high μ side every time aprescribed period of time passes; depressurization of the wheel on thehigh μ side so that the computed value of the wheel deceleration doesnot exceed a prescribed value; or delaying the stop of depressurizationof the wheel on the high μ side.

(5) It is possible that only the number of counts in the depressurizingperiod in steps 16, 25, etc., is counted or the condition of split μ isdetected depending on the presence or absence of depressurizationcontrol in these steps.

(6) While explanation was given on the select-low control of rear wheelsin the above-mentioned embodiments, the control in accordance with thisinvention is, of course, effective in the case of no select-low control.

What is claimed is:
 1. An anti-skid control apparatus for a vehiclebraking system having front wheels, rear wheels, and a brake system foroptionally exerting braking force on the wheels wherein brake fluid isemployed to transmit the braking force, the anti-skid control apparatuscomprising:a) wheel speed sensors for measuring speed of the wheelsrespectively, b) a modulator for selecting a mode for each of thewheels, the mode being selected out of an increase pressure mode, and atleast one of a decrease pressure mode and a hold mode, the increasepressure mode is for increasing brake fluid pressure, the decreasepressure mode is for decreasing the brake fluid pressure, and the holdmold is for maintaining the pressure, c) a controller for judging, withreference to threshold values, whether a wheel is about to lock based onwheel speeds, and when a wheel is about to lock, activating anti-skidcontrol for the wheel which is about to lock, by controlling theselection of modes at the modulator so that one of the hold mold and thepressure decrease mode is selected for the wheel which is about to lock,the controller having; a pressure suppression means for stepwiselyincreasing in pressure increments the brake fluid pressure of a frontwheel which is not under the anti-skid control in the condition when theother front wheel is under anti-skid control, a split μ detection meansfor counting the number of said pressure increments, and determiningthat a split μ state exists when the number of increments exceeds aprescribed value, the split μ state being defined as a condition whenthe difference between the friction coefficients of the surface beneaththe right and left wheels exceeds a predetermined value, and a brakingforce decreasing means for decreasing the braking force on the rearwheels, during the anti-skid control of the other front wheel and in thesplit μ state.
 2. An anti-skid control apparatus in accordance withclaim 1, wherein the braking force decreasing means decreases the brakefluid pressure of the rear wheels, in synchronism to the decrease of thebrake fluid pressure of the front wheel under anti-skid control.
 3. Ananti-skid control apparatus in accordance with claim 1, wherein thebraking force decreasing means decreases the braking force of the frontwheel which is not under anti-skid control together with the rearwheels.
 4. An anti-skid control apparatus for a vehicle braking systemhaving front wheels, rear wheels, and a brake system for optionallyexerting braking force on the wheels wherein brake fluid is employed totransmit the braking force, the anti-skid control apparatuscomprising:a) wheel speed sensors for measuring speed of the wheelsrespectively; b) a modulator for selecting a mode for each of thewheels, the mode being selected out of an increase pressure mode, and atleast one of a decrease pressure mode and a hold mode, the increasepressure mode is for increasing brake fluid pressure, the decreasepressure mode is for decreasing the brake fluid pressure, and the holdmode is for maintaining the pressure, c) a controlling for judging, withreference to threshold values, whether a wheel is about to lock based onwheel speeds, and when a wheel is about to lock, activating anti-skidcontrol for the wheel which is about to lock, by controlling theselection of modes at the modulator so that one of the hold mode and thepressure decrease mode is selected for the wheel which is about to lock,the controller having: a preliminary judgment means for counting aduration of decreasing pressure mode for a front wheel which is underthe anti-skid control in the condition when the other front wheel is notunder anti-skid control, and judging whether the duration exceeds apredetermined time, a pressure suppression means for decreasing thebrake fluid pressure of the front wheel which is not under anti-skidcontrol even when the duration of the decreasing pressure mode exceedsthe predetermined time if the brake fluid pressure of the front wheelunder anti-skid control is still being decreased, a split μ detectionmeans for counting the duration of the pressure decrease, performed bymeans of the pressure suppression means, and determining that a split μstate exists when the duration of the pressure decrease exceeds aprescribed value, the split μ state being defined as a condition whenthe difference between the friction coefficients of the surface beneaththe right and left wheels exceeds a predetermined value, and a brakingforce decreasing means for decreasing the brake force on the rearwheels, during the anti-skid control of the front wheel and in the splitμ state.
 5. An anti-skid control apparatus in accordance with claim 4,wherein the braking force decreasing means decreases the brake fluidpressure of the rear wheels, in synchronism to the decrease of the brakefluid pressure of the front wheel under anti-skid control.
 6. Ananti-skid control apparatus in accordance with claim 4, wherein thebraking force decreasing means decreases the braking force of the frontwheel which is not under anti-skid control together with the rearwheels.
 7. An anti-skid control apparatus in accordance with claim 4,wherein the pressure suppression means further stepwisely increases inpressure increments the brake fluid pressure of the front wheel which isnot under anti-skid control, when the brake fluid pressure of the frontwheel, which is not under anti-skid control, is not decreased by meansof the pressure suppression means, the split μ detection further countsthe number of said pressure increments, adds the number of pressureincrements to the duration of the pressure decrease, performed by meansof the pressure suppression means, and determines that the split μ stateexists when the total of the number and the duration exceeds saidpredetermined value.
 8. An anti-skid control apparatus for a vehiclebraking system having front wheels, rear wheels, and a brake system foroptionally exerting braking force on the wheels wherein brake fluid isemployed to transmit the braking force, the anti-skid control apparatuscomprising:a) wheel speed sensors for measuring speed of the wheelsrespectively, b) a modulator for selecting a mode for each of thewheels, the mode being selected out of an increase pressure mode, and atleast one of a decrease pressure mode and a hold mode, the increasepressure mode is for increasing brake fluid pressure, the decreasepressure mode is for decreasing the brake fluid pressure, and the holdmold is for maintaining the pressure, c) a controller for judging, withreference to threshold values, whether a wheel is about to lock based onwheel speeds, and when a wheel is about to lock, activating anti-skidcontrol for the wheel which is about to lock, by controlling theselection of modes at the modulator so that one of the hold mold and thepressure decrease mode is selected for the wheel which is about to lock,the controller having: a pressure suppression means for stepwiselyincreasing in pressure increments the brake fluid pressure of a frontwheel which is not under the anti-skid control in the condition when theother front wheel is under anti-skid control, a split μ detection meansfor counting the number of said pressure increments, and determiningthat a split μ state exists when the number of increments exceeds aprescribed value, the split μ state being defined as a condition whenthe difference between the friction coefficients of the surface beneaththe right and left wheels exceeds a predetermined value, and a thresholdvalue changing means for changing the threshold value corresponding tothe rear wheels, during the anti-skid control of the front wheel and inthe split μ state, so that either one of the decrease pressure mode andthe hold mold is activated easily for the rear wheels.
 9. An anti-skidcontrol apparatus in accordance with claim 8, wherein the thresholdvalue changing means changes the threshold value for judging whether toenter into the decrease pressure mode.
 10. An anti-skid controlapparatus in accordance with claim 9, wherein the threshold valuechanging means maintains, during the anti-skid control, the changedthreshold value at the start of anti-skid control.
 11. An anti-skidcontrol apparatus in accordance with claim 9, wherein the change in thethreshold value when the anti-skid control is initiated is differentfrom the change in the threshold value after initiation of the anti-skidcontrol.
 12. An anti-skid control apparatus in accordance with claim 8,wherein the threshold value changing means changes the threshold valuefor judging whether to enter into the hold mode.
 13. An anti-skidcontrol apparatus in accordance with claim 8, wherein the thresholdvalue changing means changes the threshold value corresponding to thefront wheel which is not under the anti-skid control together with therear wheels, so that either one of the decrease pressure mode and thehold mode is activated easily for the front wheel.
 14. An anti-skidcontrol apparatus for a vehicle braking system having front wheels, anda brake system for optionally exerting braking force on the wheelswherein brake fluid is employed to transmit the braking force, theanti-skid control apparatus comprising:a) wheel speed sensors formeasuring speed of the wheels respectively, b) a modulator for selectinga mode for each of the wheels, the mode being selected out of anincrease pressure mode, and at least one of a decrease pressure mode anda hold mode, the increase pressure mode is for increasing brake fluidpressure, the decrease pressure mode is for decreasing the brake fluidpressure, and the hold mold is for maintaining the pressure, c) acontroller for judging, with reference to threshold values, whether awheel is about to lock based on wheel speeds, and when a wheel is aboutto lock, activating anti-skid control for the wheel which is about tolock, by controlling the selection of modes at the modulator so that oneof the hold mold and the pressure decrease mode is selected for thewheel which is about to lock, the controller having: a preliminaryjudgment means for counting a duration of decreasing pressure mode for afront wheel which is under the anti-skid control in the condition whenthe other front wheel is not under anti-skid control, and judgingwhether the duration exceeds a predetermined time, a pressuresuppression means for decreasing the brake fluid pressure of the frontwheel which is not under anti-skid control even when the duration of thedecreasing pressure mode exceeds the predetermined time if the brakefluid pressure of the front wheel under anti-skid control is still beingdecreased, a split μ detection means for counting the duration ofpressure decrease, performed by means of the pressure suppression means,and determining that a split μ state exists when the duration of thepressure decrease exceeds a prescribed value, the split μ state beingdefined as a condition when the difference between the frictioncoefficients of the surface beneath the right and left wheels exceeds apredetermined value, and a threshold value changing means for changingthe threshold value corresponding to the rear wheels, during theanti-skid control of the front wheel and in the split μ state, so thateither one of the decrease pressure mode and the hold mode is activatedeasily.
 15. An anti-skid control apparatus in accordance with claim 14,wherein the threshold value changing means changes the threshold valuefor judging whether to enter into the decrease pressure mode.
 16. Ananti-skid control apparatus in accordance with claim 15, wherein thethreshold value changing means maintains, during the anti-skid control,the changed threshold value at the start of anti-skid control.
 17. Ananti-skid control apparatus in accordance with claim 15, wherein thechange in the threshold value when the anti-skid control is initiated isdifferent from the change in the threshold value after initiation of theanti-skid control.
 18. An anti-skid control apparatus in accordance withclaim 14, wherein the threshold value changing means changes thethreshold value for judging whether to enter into the hold mode.
 19. Ananti-skid control apparatus in accordance with claim 14, wherein thethreshold value changing means changes the threshold value correspondingto the front wheel which is not under anti-skid control together withthe rear wheels, so that either one of the decrease pressure mode andthe hold mode is activated easily for the front wheel.
 20. An anti-skidcontrol apparatus in accordance with claim 14, wherein the pressuresuppression means further stepwisely increases by pressure incrementsthe brake fluid pressure of the front wheel which is not under anti-skidcontrol, when the brake fluid pressure of the front wheel which is notunder anti-skid control is not decreased by means of the pressuresuppression means, the split μ detection further counts the number ofpressure increments, adds the number of pressure increments to theduration of the pressure decrease, performed by means of the pressuresuppression means, and determines that the split μ state exists when thetotal of the number and the duration exceeds said prescribed value.